Reconsidered: should vs. must

The purpose of this study is to distinguish between the usages of the epistemic modals should and must based on the deductive processes and logical inferences that they indicate. To achieve this purpose, the study presents several examples using should and must from previous studies, which have recognized differences in the meanings of the terms especially in relation to time and causality. However, it is found that some of these examples cannot be explained by previous approaches applied separately. Instead, the approaches must be applied together in the case of examples belonging to one of four categories. First, epistemic modal must can be used when a deduction is being made about a present or past situation and causes are being inferred from consequences. In contrast, in the second category, with should , deducing the time of the event is only possible in the future tense, when it is integrated with inferred consequences from their causes. However, in the last two categories, both should and must can be used when the inferred time of the event is in the present or the past and causality moves from cause to consequence, or when the inferred situation is in the future and the causality moves from consequence to cause.